Is there any update on this? It appears this is becoming a common breaking point for new installations. 
The latest victim is using an HP All-in-One, who once again got their newly installed system running after applying the above three changes.
So it’s not occurring only with laptops. It’s not pretty to have to walk users through booting into a live USB to make the corrections in a manjaro-chroot jail. (“First impressions” of Manjaro comes to mind, and the installation was supposedly “successful”, until they’re met with a jarring surprise when they reboot.)
As @linux-aarhus alluded, this would have to be a patch against /etc/mkinitcpio.conf and /etc/default/grub, since neither of those files are in Manjaro’s GitLab, yes?
This is not something that can be done by Calamares towards the end of the installation process?